Comparative XPS Study of Silver and Copper Surfaces Exposed to Flowing Air Containing Low Concentration of Sulfur Dioxide

Sulfur dioxide (SO2), a typical gaseous compound found in the atmosphere, corrodes metals, particularly outdoors. When SO2 adsorbs on the surface electrolyte, it easily dissolves, resulting in the formation of protons (H+) and bisulfite ions (HSO3). The former reduces the pH of the surface electrolyte, which promotes metal dissolution. Silver and copper are commonly used in the electronic components and devices typically found in telecommunication equipment. Since silver and copper are affected by SO2, it is important to know the effects of SO2 on their corrosion behavior to predict the life expectancy of the equipment. Such knowledge would also be useful for coping with anomalous situations, such as volcanic eruptions, in which the SO2 concentration can exceed several hundred ppb. A previous study showed that silver and copper plates exposed to the atmosphere were severely corroded following the volcanic eruption on Miyake Island in 2000.1) Much effort has been expended on clarifying the effects of SO2 on atmospheric silver and copper corrosion. A number of laboratory experiments have been reported regarding the effects of exposure to high SO2 concentrations.2)-15) Using infrared reflection absorption spectroscopy (IRAS), Persson and Leygraf showed that sulfite formed on the surface of copper during the initial stage when it was exposed to 0.21 ppm SO2 in flowing air at 80% relative humidity (RH).16) In situ analysis using IRAS and a quartz crystal microbalance by Ito et al.17)-18) revealed that sulfite-like CuSO3Cu2SO3・2H2O (Chevreul’s salt) and sulfate CuSO4・ 5H2O formed as corrosion products on copper exposed to flowing air containing 10 ppm SO2 at 80% RH. Moreover, the formation rate of sulfite was higher than that of sulfate in the very early stage.17)-18) In contrast, the sulfidation rate by SO2 for silver is much lower than that by reduced sulfur species, such as H2S and COS.12) Since SO2 is considered to be a weak corrosive agent for silver, the effect of SO2 on atmospheric silver corrosion in the initial stage has received little attention. In this study, the chemical state of the sulfur was investigated when silver and copper plates were exposed for one hour to flowing air containing low concentrations of SO2. Since the electrochemical reactions occur at the metal/ electrolyte interface in the early stage of corrosion, X-ray photoelectron spectroscopy (XPS) was used. The SO2 concentrations were 18 and 148 ppb ; the former is at least one order of magnitude lower than those in previous studies.16)-18)